Glucagon-like peptide-1 and beta cell glucose sensitivity - a glucose ramp study in mice

The incretin glucagon-like peptide-1 (GLP-1) is a gut hormone but also locally produced in pancreatic islets. We evaluated effects of GLP-1 on the insulin response to a gradual increase in glucose in mice within physiological levels. We initially developed a glucose ramp technique in mice. Glucose levels were slowly increased by 0.2 mmol/l/min for 40 min under control conditions, during intravenous infusion of GLP-1 and in GLP-1 receptor knockout mice. In control mice, glucose levels increased from 8.5 ± 0.3 to 16.1 ± 0.3 mmol/l over the 40 min, i.e., by 0.22 ± 0.01 mmol/l/min. This resulted in a slow increase in insulin levels by 96 ± 38 pmol/l from the baseline of 319 ± 53 pmol/l. GLP-1 at 0.5 nmol/kg as bolus plus 0.3 nmol/kg/min over 40 min progressively increased this insulin response by 100-fold, to 9.5 ± 0.2 nmol/l (P < 0.001). Higher doses of GLP-1 enhanced the insulin response similarly (1.0 or 3.0 nmol/kg bolus followed by 0.4 or 1.2 nmol/kg/min), whereas a lower dose (0.3 nmol/kg bolus plus 0.15 nmol/kg/min) had no significant effect compared to controls. Moreover, there was no significant difference in insulin responses between controls and GLP-1 receptor knockout mice. Since the increase in glucose levels were standardized, there was no significant difference in glucose levels between the experimental groups. We conclude that the glucose ramp technique is a tool for studies on insulin responses to slow changes in circulating glucose levels in mice. We also conclude that GLP-1 is extraordinarily potent in enhancing the insulin response to a slow increase in glucose levels.